Wind turbines are used to produce electrical energy using a renewable resource and without combusting a fossil fuel. Generally, a wind turbine converts kinetic energy from the wind into mechanical energy and then subsequently converts the mechanical energy into electrical power. A horizontal-axis wind turbine includes a tower, a nacelle located at the apex of the tower, and a rotor that is supported in the nacelle. The rotor is coupled with a generator for converting the kinetic energy of the blades to electrical energy.
Generators for wind turbines are relatively large, heavy rotating electrical machines having an outer housing, a drive shaft rotatably mounted within the outer housing, and a stator assembly and rotor assembly also positioned within the outer housing. The outer housing may include a generally cylindrical outer side wall that is closed off at each end with an end shield. The drive shaft is operably coupled to the wind turbine rotor, either directly or through, for example, a gear box, such that rotation of the rotor causes a rotation of the generator drive shaft. In conventional generator constructions, the stator assembly is coupled to the outer housing so as to be stationary, and the rotor assembly is operatively coupled to the drive shaft so as to be rotated with rotation of the drive shaft. Relative rotation between the stator and rotor assemblies produces electricity.
The drive shaft is supported in the generator housing by a pair of bearings, typically at respective ends of the generator and positioned adjacent the end shields. The bearings are configured to limit the movement of the drive shaft (and thus rotor assembly) against axial thrust, radial displacement, and tilt (e.g., 5 of the 6 degrees of freedom). In essence, the bearings constrain the drive shaft to a single degree of freedom, i.e., rotation, which they strive to provide without significant losses or overheating. One of the bearings is typically axially fixed to the housing so as to limit movement against axial loading. This is referred to as a fixed bearing. The other bearing, however, is not axially fixed but is permitted a limited amount of axial displacement relative to the housing. This bearing is referred to as a floating bearing and is provided so as to prevent the generator's moving parts (e.g., drive shaft) from becoming over constrained by the generator's stationary support system for those moving parts (e.g., generator housing).
In this regard and by way of example, the moving and stationary parts of a generator typically operate at different temperatures and are made of different materials having different thermal expansion coefficients. Accordingly, the moving and stationary parts may be subject to different amounts of thermal expansion/contraction. If the generator housing or other associated supports over-constrain the drive shaft, such as by axially fixing both bearings, then the thermal expansions/contractions of the generator become restricted, resulting in large axial, radial and tilt forces being imposed on the bearings. This, in turn, leads to excessive vibrations, overheating, and ultimately, premature bearing failure, thus prompting frequent and costly replacement. As noted above, to avoid over-constraining the drive shaft, one of the bearings is configured as a floating bearing such that, for example, the effects of thermal expansions/contractions may be readily accommodated via a limited amount of axial movement of the floating bearing. Configuring one of the bearings as a floating bearing extends the working life of the bearings, thereby reducing maintenance costs and increasing production time.
While the floating bearing works for its intended purpose, there are some drawbacks to the implementation of the floating bearing in current generators for wind turbines. In this regard, the floating bearing may be configured as a rolling element bearing having, for example, an inner race coupled to and rotatable with the drive shaft, an outer race coupled to the stationary housing in a manner that permits axial movement, and rolling elements, such as ball bearings or rollers, disposed between the inner and outer races. More particularly, the floating bearing is positioned adjacent an end shield such that there is a slight interference fit between the outer periphery of the outer race and the inner periphery of the end shield. This region between the outer periphery of the outer race and the inner periphery of the end shield is critical in floating bearing designs and may be sensitive to tolerances and clearances. For example, should the clearances be too tight, the bearing may become locked as a result of, for example, thermal expansions/contractions. On the other hand, excessive clearances may allow the bearing to tilt or cock in a locked position or subject the generator to excessive vibrations.
Upon assembly of the generator, a layer of lubricant is generally disposed in the critical region between the outer periphery of the outer race and the inner periphery of the end shield. This lubrication layer facilitates the axial movement of the floating bearing during operation of the generator, as discussed above. With extended use of the generator, however, this lubrication layer may decompose, be scraped off, or weep out of the space between the bearing and the end shield. Unfortunately, current generator designs provide no mechanism or provision for replenishing the lubrication layer in this region. Thus, with the loss or reduction of the lubrication layer, the relative movement between the bearing and the end shield may become restricted. In some circumstances, the floating bearing may become jammed, cocked or otherwise locked such that it loses its ability to move axially. In this case, the floating bearing operates like a fixed bearing. Since both bearings are now axially fixed, the drive shaft may become over-constrained by the generator housing, potentially resulting in premature failure of one or both bearings.
Accordingly, there is a need for a generator for a wind turbine that provides improved bearing lubrication. More particularly, there is a need for a generator that provides for replenishing the lubrication layer in the region between the floating bearing and the housing, thereby providing continued axial displacement of the floating bearing during use. There is also a need for a wind turbine having a generator with improved bearing lubrication and a method of operating a generator for a wind turbine so as to have improved bearing lubrication.